Pick one - rebooting can be narrowed down to:
Heat
Faulty RAM (run
Memtest86)
Faulty hardware
Conflicting drivers
Drivers needing updating
BIOS needing updating
Grounding issues with motherboard and case
Insufficient and/or failing power supply
etc
To start off, I would check your event logs. Go to START menu, select RUN, and type
eventvwr.msc followed by return. Look in both the SYSTEM and APPLICATION logs and see what shows up at the time of reboots.
I would also try and get ahold of a STOP code rather than have your PC reboot. Right-click MY COMPUTER and select PROPERTIES. Click on ADVANCED tab. Under STARTUP and RECOVERY, click on SETTINGS. Under SYSTEM FAILURE, uncheck
automatically restart. This will provide you with a blue screen (BSOD) and an error code the rather than restarting next time the error/conflict occurs.
Faling those tips, we can start tackling the other possible culprits one by one.
EDIT: Typo.